A refrigerator which uses any kind of cold accumulator such as a Stirling type, a GM type (Gifford-MacMahon type), or a pulse tube type needs a cold accumulator filled up with cold accumulating material in order to increase the refrigerant efficiency of refrigeration. These cold accumulators take heat from a compressed operating gas which flows in one direction, accumulate the heat, and transfer the accumulated heat to an expanded operating gas which flows in another direction.
Conventionally, an alloy of copper or lead, etc., is mainly used for a cold accumulating material filled up in the cold accumulator. However, since the specific heat of copper or lead is due only to lattice vibration, the specific heat is large at temperatures above 40 K but extremely small at low temperatures below 20 K.
As a result of this, when a cold accumulator filled up with such cold accumulating material is used in a refrigerator (more particularly multiple type refrigerator), the cold accumulator cannot fully absorb the heat from the compressed operating gas, and also cannot fully transfer the heat to the expanded operating gas. A result of this is that the cold accumulator filled up with the cold accumulating material cannot reach extremely low temperatures.
Cold accumulators proposed to solve the problem described above are disclosed in Japanese Laid-Open Patent Application No. 1(1989)-310269, European Patent No. 327293 B1 and U.S. Pat. No. 5,186,765, the disclosures of which relating to use of cold accumulating material in low-temperature refrigeration equipment are hereby incorporated by reference. As the typical example, the cold accumulator is filled up with cold accumulating material of a magnetic body made from Er.sub.3 Ni. The specific heat of Er.sub.3 Ni is due to lattice vibration and spin vibration. This specific heat is larger than the cold accumulator material made of copper or lead at extremely low temperatures below 20 K (more particularly under 10 K), so that these cold accumulators can improve the efficiency of accumulating heat.
However, the cold accumulator material made of Er.sub.3 Ni has a magnetic critical point (a phase transition between magnetic states) at about 8 K. Therefore, while the specific heat is large at temperatures under 10 K, it is small in the temperature range 10 to 30 K. As a result of this, though the efficiency of accumulating heat is large at temperatures under 10 K, it is not enough in the temperature range 10 to 30 K. Accordingly, cold accumulator material made of Er.sub.3 Ni is less efficient for refrigerators which generate refrigeration in the temperature range 10 to 30 K.